Switch mode power supplies which operate off of an AC line generally have a passive filter and a rectifier which charges a subsequent large charge storage capacitor which in turn shunts the input to the subsequent switching circuitry of the power supply. Current input to the rectifier tends to occur in short pulse-like waveforms with high peak values which are very short in duration compared to the period of the substantially sinusoidal waveform of the input AC voltage. This is due in part to the acquired voltage of the charge storage capacitor back biasing the rectifying diodes during a large fraction of the input voltage waveform period whereby current is drawn from the AC line only during the peak portions of the AC voltage waveform. This pulsed input current has a high RMS current component and is rich in odd harmonics resulting in a poor power factor at the input of the power supply.
Techniques to improve the power factor of a power supply operated off of an AC line have included many approaches to reduce the harmonic components of the current such as utilizing passive low frequency input filtering which is designed to attenuate specific low frequency odd harmonics of the input current. Due to the need to provide individual filters to filter each low frequency odd harmonic, the magnetic components required add substantial bulk and weight to the power supply and are only operative at very specific harmonic frequencies. Their use is not desirable in many applications because of size and weight considerations.
In one particular power factor correction scheme disclosed in U.S. Pat. No, 4,369,490 a wave transforming network is connected at the AC side to a power rectifier. It is operative to convert the input sinusoidal signal waveform to a trapezoidal waveshape. The circuit components of this network include a storage inductance and a storage capacitor in which the two components are resonant at a frequency greater than two but less than three times the frequency of the input AC sinusoidal signal. The storage inductance and the storage capacitor are connected in a T arrangement in some embodiments and a series arrangement in others. Both arrangements are asserted to reduce harmonics and improve power factor at the input. A parallel resonant connection of the storage inductance and the storage capacitor is noted in this patent as a prior art arrangement that may be tuned to filter a specific harmonic. This arrangement is professed to be unsatisfactory because a separate filter is needed for each specific frequency of the harmonics that are suppressed.
Active control of the current wave shape can be used to control the input impedance to approximate a resistive load at the input and produce a very high power factor at the input. This active control may be provided by a boost type converter. This boost type converter typically includes a regulatory feedback control to control the duty ratio of its power switch. This converter is inserted between the rectifier and the storage capacitor and regulated to control the current waveform. A subsequent power converter is used to achieve the desired line and load regulation. This arrangement however adds considerable overhead expense to the power supply and the high parts count incurred with the added circuitry also significantly reduces its reliability.